# 2 refrigerators maintaining temperature

1. Jun 28, 2009

### redsrtturbo

I can't go to sleep until I find an answer and I'm no student of physics so I was hoping for a little help.

If two refrigerators are at the same thermostat setting. One empty (assumed regular Earth air inside) and one full of food.

Both have already reached the temperature on the thermostat. Which will require more energy to maintain at the set temperature (not specified)?

I don't know what equations to use here or if my thoughts are even in the right direction but here goes:

It would take more energy to maintain the fridge full of food b/c it has more matter that is subject to reaching equilibrium and so the refrigerator has to actively fight this and costs more energy than the fridge full of air.

Hit or Miss?

Last edited: Jun 29, 2009
2. Jun 29, 2009

### Mapes

Why would a refrigerator require more energy to maintain the contents' temperature after reaching the setpoint? Make a list of the inefficiencies of the system. Do any of them involve the contents (air or solid)?

3. Jun 29, 2009

### redsrtturbo

They would be set in the real world.

One would be full (fluids and solids can be assumed), and the other just plain air.

I assume they would still have to cool to remain at an arbitrarily set temperature. Which would have to cool more (use more energy) to keep the temperature inside at the predetermined level.

4. Jun 29, 2009

### Mapes

Again, we have to look at why the refrigerator has to keep running at all. Is it heat conduction through the walls, for example? This factor doesn't seem to depend on whether the refrigerator is full or empty.

5. Jun 29, 2009

### Blenton

They would be the same provided you didn't open the fridge (in that case the fridge with food would be cooler than the one with just air since the solid matter wouldn't disperse out the door).

6. Jun 29, 2009

### redsrtturbo

ok- got the answer..... the fridge would only have to run to maintain temp, kinda like when you hear them cycle on. I can't believe such a simple answer was so elusive.

Thanks, now I can get some sleep.

7. Jun 29, 2009

### nnnm4

Well, there are more items in the full fridge that radiate energy, so would it actually require slightly more energy?

8. Jun 29, 2009

### redsrtturbo

that's what I thought- since there was more matter in the fridge to transfer heat, it would heat up more and so need to be cooled more-

9. Jun 29, 2009

### Andrew Mason

A refrigerator with food in it would draw more energy to keep it cool because people would be opening the door to get stuff to eat. If it was empty, no one would bother opening the door. But if this a physics and not a home economics question, that is not what is being asked.

Mapes is right. Since the food is already at the temperature set by the thermostat, the only thing which determines how much energy the refrigerator uses would be the amount of heat entering the refrigerator from the outside. That is independent of the amount of food in the fridge (if the door is kept closed).

AM

10. Jun 30, 2009

### Redbelly98

Staff Emeritus
Just to follow up on this. The refrigerator with food in it would require more energy to reach the temperature setting of the thermostat.

But, that is irrelevant to answering this question, since it asks about the energy used after it has reached the temperature setting.